their Structure and Affinities. 67 



The coralliim of the Anthozoa differs so widely from the polypary 

 of the graptolite, that this great division of the Coelerderata may at 

 once be excluded. And I do not except from this exclusion the 

 Pennatulidce to which Beck in Murchison's ''Silurian System" re- 

 ferred the graptolites. The apparent resemblance between Diplo- 

 grapsus and Pennatula or Virgularia is very superficial. The three 

 genera may agree in being free organisms, and they have a bilateral 

 arrangement of parts and a prolonged solid axis, but in the fossil the 

 axis is slender and corneous, and is produced at the distal end of the 

 organism, while in the recent genera it is thick and calcareous, and 

 proceeds from the proximal end. The cells containing the animals 

 are dug out of the coenosarc in Pennatula and Virgularia, while in 

 the graptolites the polypary is corneous and external. Our com- 

 parison will therefore be confined among the Ccelenterata to the 

 Hydrozoa. 



In the same way we may strike off those forms of the Polyzoa 

 which have obviously no resemblance to the graptolites. In dealing 

 with this class it is fortunate for the paleeontologist (although a matter 

 of regret to the zoologist) that its classification is based almost entirely 

 on the structure of the polyzoary. The freshwater forms and those 

 without a hard cuticular layer may at once by set aside. In both the 

 Cheilostomata and Cyclostomata the cells are budded from each 

 other, and eventually all living connection between the polypites is 

 cut off, so that in adult specimens no opening has been detected 

 between neighbouring cells. The presence of a canal, common to 

 and opening freely into all the cells, in the graptolite is sufficient 

 to exclude these orders from our consideration. The flat double 

 corneous disc of Dichograpsus has only an external resemblance to 

 the discoid calcareous polyzoary of Defrancia, and in every other 

 respect the structure of the two genera is totally different. The 

 only order of Poly%oa with which the graptolite can be compared is 

 that named by Busk Ctenostomata, and by Gray Polyzoa cornea from 

 the composition of their polyzoary. The members of this order have 

 a common basal tube on which the individual cells are placed. The 

 base of the cell is cut off from the common tube by a septum, but 

 through this there is a small opening by which the various poly- 

 pites have a living connection with the circulation and the structures 

 in the tube which are common to all the individuals of the colony.^ 

 The correspondence in the arrangement of the cells on a common 

 canal in Vesicidaria and Graptolithus, or in Farrella and Rastrites is 

 very obvious, but the existence of the perforated septum at the base 

 of the cell is an invariable and important character in all these 

 Pohjzoa which at once distinguishes them from the graptolites. 



Professor Huxley in his Monograph of the Oceanic Hydro.oa, pub- 

 lished by the Eay Society, has grouped the Hydrozoa into six orders 



^ Dr. Nicholson summarily dismisses the Tolyzoa by asserting that they *' have, as 

 a rule, a more or less calcareous test, and the individuals forming the compound 

 organism are not united by any organised connecting substance." Perhaps a common 

 circulation in the basal tube is not, in Dr. Nicholson's opinion, an " organised 

 connecting substance," and Gray and Busk may be ignorant of the true nature of the 

 Polyzoan test. 



